The increasing use of high frequency electromagnetic radiation in radar and communication fields has resulted in the need for materials suitable as radiation absorbers, reflectors, filters and polarizers. Of particular interest are materials which can be impedence matched to the transmission medium and used as covers or outer layers of objects to reduce the radar reflectivity of the objects. For example, there is extensive interest in the use of radar absorbing materials to reduce the radar cross-section of military hardware such as aircraft, missiles, tanks and ships.
The use of radar defeating sheet material is known in the prior art. It has been recognized that conductive fibers can be incorporated in yarns which are knitted into camouflage material to provide a radar reflectance characteristic similar to the surrounding environment. Such a material is disclosed in U.S. Pat. No. 4,064,305 to Wallin. Elsewhere, metallized sheet-form textile materials or parallel metal wires have been disclosed as reflection and polarization control media for microwaves. See U.S. Pat. Nos. 4,320,403 to Ebneth et al and 4,400,701 to Dupressoir. A later Ebneth et al patent (U.S. Pat. No. 4,439,768) discloses the use of multiple layered fabric materials in microwave screening applications in which some of the sheet form material is metalized. Finally, U.S. Pat. No. 4,433,068 to Long et al teaches the use of apparently amorphous polyimide microballons foam with filler to improve microwave absorbing properties. Long et al state that the microwave absorption of polyimides can be modified and improved by the addition of from about 1 to 50 weight percent microwave absorbing material such as graphite powder, ferrites, metal-ceramic compounds such as ferro titanate or mixtures thereof.
There are two commonly used methods of making impedance matched structures. The simplest is to use non-magnetic materials with as low a dielectric constant as possible. If these materials are also employed in a low density structure (such as a foam), the dielectric constant will approach one. The problem with such materials is that they have almost no ability to absorb the incident radiation, and thus will not significantly reduce the reflection from metallic objects which might be behind the low dielectric constant material. The second method for achieving some degree of impedance matching is to use magnetic insulators such as ferrites. These materials can have reasonably high magnetic permeability and electric permitivity as well as significant absorption mechanisms. The major problems with these materials are that they are heavy, their magnetic permeability is frequency dependent and they work best at low microwave frequencies, i.e., at frequencies less than 10 GHz.
In addition, the dielectric constant of such materials is often significantly higher than the magnetic permeability at frequencies of interest. This is primarily because the permeability of the materials decreases rapidly with increasing frequency, while the dielectric constant varies less rapidly with frequency.
Accordingly, it is an object of the present invention to provide a flexible sheet material having a tuned response to high frequency electromagnetic radiation.
It is another object of the present invention to provide impedance matched sheet material having a preselected magnetic permeability and dielectric constant.
It is another object of the present invention to provide a material tuneable to a desired impedance.
These and other objects and features of the claimed invention will be apparent from the following written description and claims, considered with the drawings herein.